Hematite films

Oxide films can be stripped off iron by using bromine in methanol followed by heating at 300 °C in N2 to remove FeBr2 (Mayne and Ridgeway, 1971). The thickness of such films can be measured by weighing, by cathodic reduction and from the interference colours of the films the latter technique can also be applied to measurement of film thickness in situ. The first order interference colours of hematite films on iron are yellow/brown, mauve, blue and silver grey and the second order colours are pinky-blue, blue and greenish-blue (Evans, 1963). 

The promising initial study of hematite nanorod initiated a study of hematite nanorods for the aim of water oxidation [29]. IPCEse and IPCEee in a three-electrode set-up were determined at 350 nm to 2.3 % and 1.0 %, respectively. The fact that IPCEse is twice the IPCEee reveals that the collection of the photogenerated electrons across the hematite film is poor. Nevertheless, the ratio in the quoted investigation [29] is significantly smaller compared to the nanosized isotropic random walk hematite particle system reported by Bjbrksten et al [44]. The photocurrent density at an intensity of 1 sun was in the (xA/cm2 scale in 0.1 M NaOH. 

Bjorksten U, Moser J, Gratzel M (1994) Photoelectrochemical studies on nanocrystalline hematite films. Chem Mater 6 858-863... 

Fig. 4.7 Hematite films created by USP show a platelet morphology by cross-sectional SEM analysis (top, fi om [101]), and an increased IPCE at longer wavelengths compared to films created by conventional spray pyrolysis (bottom, from [94], used with permission)...

Fig. 4.8 Morphology of Si-doped hematite films created by the APCVD technique as observed by SEM. The left side shows a cross-sectional view of the Fe203, prepared at a substrate temperature of 420°C (heater setpoint of 470°C), on top of the F Sn02 substrate. The right shows top-down views of films prepared with heater set points of450°C above) and 490°C below). From [13] used with permission...

Basic) Magnesium phosphate Magnesium hydroxy-phosphate 3Mg3(P04)2-Mg(0H)2 Forms with hematite in passivated films at lower temperatures. 

Ferric phosphate FeP04 Found in low temperature passivation film with hematite. 

For iron oxides, IR spectroscopy is useful as a means of identification. Hematite crystals in films that were too thin (<70nm) to be characterized by XRD were shown by IR to be oriented with the c-axis perpendicular to the surface of the film (Yubero et al. 2000). This technique also provides information about crystal morphology, degree of crystallinity and the extent of metal (especially Al) substitution because these properties can induce shifts in some of the IR absorption bands. It is also widely used both to obtain information about the vibrational state of adsorbed molecules (particularly anions) and hence the nature of surface complexes (see Chap. 11) and to investigate the nature of surface hydroxyl groups and adsorbed water (see Chap. 10). Typical IR spectra of the various iron oxides are depicted in Figure 7.1. Impurities arising either from the method of preparation or from adsorption of atmospheric compounds can produce distinct bands in the spectra of these oxides -namely at 1700 cm (oxalate), 1400 cm (nitrate) and 1300 and 1500 cm (carbonate). 

The oxide film formed in dry air at room temperature consists of a spinel phase, probably a solid solution of magnetite and maghemite. Such films form on magnetic tapes. They are around 1.5-2.0 nm thick, and in a dry atmosphere, can provide indefinite protection (e.g. the Delhi pillar). Ali and Wood (1969) found that with time and at a relative humidity of 46%, some hematite developed as well. At higher temperatures (200-300 °C) well defined duplex films with an inner layer of magnetite... 

Thin films of hematite are grown on silicon wafers by ion beam induced, chemical vapour deposition (IBICVD). FeCOs vapour is passed over the silicon substrate which is bombarded by OJ ions which decompose the iron compound (Yubero et al., 2000). 

Beermann, N. Vayssieres, L. Lindquist, S.E. Hagfeldt, A. (2000) Photoelectrochemical studies of oriented nanorod thin films of hematite. J. Electrochem. Soc. 147 2456-2461... 

Beermann N, Vayssieres L, Lindquist SE, Hagfeldt A (2000) Phtotelectrochemical studies of oriented nanorod thin film of hematite. J Electrochem Soc 147 2456-2461... 

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